Seed bank composition varies along invasion and inundation gradients in vernal pool wetlands

Abstract

Environmental filters are well-known influences on aboveground vegetation community structure; however, less is known about their role on belowground seed banks. Understanding the influence of environmental filters on the composition of seed banks can reveal community dynamics across known environmental gradients and facilitate restoration efforts. We examined the influence of environmental filters on seed banks of vernal pools by characterizing seed density and diversity along seasonal inundation gradients. We also sampled seed banks from both naturally occurring and restored vernal pools that differed in their aboveground communities (invasive or native species dominated) in a long-term field study in Solano Co., California, USA. We found that the highest seed densities were associated with the longest inundation period and in the naturally occurring pools. Inundation gradients within a pool had little influence on seed bank diversity, yet among the pool types diversity and community metrics varied. The naturally occurring pools, regardless of invasion status, displayed a greater species richness and diversity than constructed pools. Our greenhouse germination trials did not show a strong relationship of inundation depths influencing species and total germination. Overall, we found that local position in the field along inundation gradients within a pool strongly affected soil seed bank density, while seed bank diversity varied more across pool types. Environmental filters may be limiting germination with the pool bottoms having the highest inundation and maintain the densest seed bank, but our lack of difference in the germination trials suggests alternative mechanisms other than inundation may be hindering germination.